Combustion engines have been used for years in various transportation applications. These engines offer high power output in a relatively small space and give good response to power changes. However, existing engine efficiency varies with the output level of the engine. The engines exhibit poor efficiency at low power but the efficiency improves as load increases until a maximum is reached at the design output point of the engine. Large amounts of heat generated by the burning fuel are lost in the exhaust system and in the cooling process required to maintain safe working temperatures of the metal parts. Thus, there is a need to recapture and use this wasted heat energy and improve efficiency over the entire load range of the engine.
A number of inefficiencies exist in present combustion engines because of their basic structure and operation. Due to a limited amount of time between cycles, fuel is added quickly to compressed air before ignition. There is a need for the fuel to be injected earlier or over a longer period of time so a proper stoichiometric mixture of air and fuel may be had. Also, the expansion rate is affected by the rigid connection of the piston to the output of the engine. This combination of events can cause unwanted high pressure and transient temperature spikes that can result in excessive pollution emissions and noise.
Many attempts at increasing efficiency at the design rating of combustion engines have been made. However, the design horsepower of an engine powering a vehicle such as an automobile is seldom used. The normal driving cycle of an automobile generally requires minimum engine horsepower, a point where poor efficiency will exist even in the advanced engine development concepts presently underway. Under-powering the vehicle has been a ready, although undesirable, alternative way to increase efficiency.
It is expected that the adiabatic engine will provide the most thermally efficient cycle with the majority of the heat of fuel being transferred into mechanical work and exhaust gas. Isolating each step of the combustion process is considered to be the best method to create an adiabatic engine. One such attempt is the engine of the "Brayton" type, i.e., one employing separate compressors and combustion chambers. The separate air compressor is used for receiving and compressing a flow of air to a given pressure to provide compressed air to a separate point where combustion takes place. Many United States patents teach compression external to the combustion area; see, for example, U.S. Pat. Nos. 713,366; 724,239; 1,038,970; 1,111,841; 1,156,534; 3,811,271; 4,040,400; 4,230,075; 4,300,486; 4,333,424; 4,369,623; and 4,592,309. In order to maximize efficiency, some of these applications attempt to extract heat during compression of the air in order to provide a denser air charge in the combustion cylinder. The engines shown in these patents use a multi-stage compressor which is not the most adequate solution to achieving maximum efficiency, because it does not remove heat immediately as pressure is increased.
The majority of existing combustion engines compress the air and burn the fuel in the sat-ne cylinder. This operation causes the fuel to burn while the piston is moving, which results in less expansion for all of the burning fuel and therefore reduces efficiency. A thermodynamic advantage that has been employed to avoid this problem is positioning the combustion chamber external to the expansion or positive displacement chamber. This permits a greater independence of the fuel and combustion temperature used and allows greater flexibility in ignition timing. In order to maximize efficiency, expansion must continue to as low a temperature as possible. Present combustion engines typically do not provide expansion to a temperature as low as possible. In addition to allowing maximum expansion, there is a need to use separate combustion and expansion chambers to avoid high heat conduction from the chambers to other parts of the engine. As the fuel burns in the combustion chamber, the highest temperatures in the cycle are presented to the metal parts. The total surface area of the combustion chamber exposed to these temperatures as the fuel is burning has a direct bearing on the amount of heat transferred to the metal parts, which in turn affects the efficiency of the engine. Present engines that burn the fuel in the same chamber as expansion occurs present a large metal area to the hot burning fuel.
Another attempt towards operating an optimally functioning combustion engine is to maintain constant or near constant pressure in the combustion chamber throughout all operating ranges. Most attempts to achieve this constant pressure are by use of a variable volume combustion chamber, which causes the compression ratio to vary. An effort at providing these variable volume combustion chambers is disclosed in U.S. Pat. No. 4,890,585. The volumes of combustion chambers are adjusted by mechanical controls and/or suitable electro-mechanical controls. However, the present systems do not limit pressure/temperature spikes or allow for control of the air per cycle; therefore, the air-to-fuel ratio must be varied to vary the power. There is a need to control the volume of air per cycle to increase the expansion ratio to a maximum and lower the peak temperatures of the engine to reduce emissions. There is also a need to extend the expansion ratio so that the hot gases may be expanded to an optimum temperature level. Such expansion would reduce operating temperature of the engine because the energy removed from the hot gas will equal that of an engine with a lower expansion ratio operating at a higher operating temperature.
There is a need for an engine capable of operating at a nearly constant air-to-fuel ratio which would allow use of maximum fuel and minimum air at low power to maximize efficiency. The reduced efficiency at low power operation in an combustion engine is due to the manner in which it operates. When an engine with a fixed volume at top dead center is operated at a fractional load, the compression ratio is reduced and/or the air to fuel ratio is increased to limit the power output. This is done mainly by throttling the air to limit its flow during the intake process or by lowering the amount of fuel used. In the first instance, the compression ratio is reduced which also reduces the expansion ratio and the efficiency of the engine; in the second instance the amount of air flowing through the engine at minimum power is almost the same as if the maximum fuel was being used. This causes low combustion temperatures resulting in incomplete burning of the fuel. Therefore, the friction and air compression losses remain nearly constant from minimum to maximum power. These losses use a significant percentage of the power produced by the burning fuel at low power output. Thus, there is a need for either the compression ratio or the air to fuel ratio to stay constant and a need to reduce friction losses of sliding parts so that a larger percentage of the power produced by the burning fuel will be converted to useful work.
Present engines exhibit excessive pollution during cold startup. This is due to the fact that catalytic convertors in the exhaust system require high temperatures to work properly; thus, the engine must run for some time before the exhaust gas heats the elements of the catalytic convertor to operating temperature. The convertors also must be of sufficient size to handle the volume of low pressure exhaust gas. Due to this large size requirement, more of the rare elements used in the convertors are needed to cover the area over which the exhaust gases must pass. This increases the cost of the convertor. Thus, there is a need for the catalytic convertor to be located in the engine system at a point where gases are at a higher temperature and lesser volume to reduce cost, improve operation, and limit the quantity of rare earth elements used.
Existing two stroke combustion engines utilize the outward stroke of the piston to pressurize an air storage space at a pressure slightly above ambient. This compressed air is used to blow out the exhaust from the previous power stroke and fill the cylinder with fresh air. The two stroke engine is affected by friction losses to a lesser degree than a four stroke engine because the friction of the rings moving against the cylinder wall is present whether the piston is moving in or out of the cylinder. In the two stroke engine the affect of this compression on the downward stroke is minimal because the air is only slightly compressed. Thus, the two stroke engine design maximizes the use of the piston moving back and forth in the cylinder to increase efficiency of operation. There is a need to better utilize this design in the modem day combustion engines.
In the operation of present day combustion engines, there is a conflict between the need to lower peak- temperatures of the system to reduce emissions and the need to raise peak temperatures to increase efficiency. The peak temperatures in the combustion engine contribute to the amount of pollution emissions emitted in the engines exhaust, especially NOx. The higher the peak temperature the higher the emission levels. Although lowering the peak temperature may reduce emissions, it also reduces efficiency of the present day engine. In order to provide maximum efficiency, present combustion engines must run at high compression ratios. These high compression ratios cause the compressed air to reach extremely elevated temperatures, and when the fuel burns, the temperature of the burned gas is elevated even further. Thus, there is a need to lower the compression temperature and peak temperature to reduce emissions while at the same time provide maximum efficiency of operation.
Another need in present combustion engines is a wider range of valve control. Present engines operate over a wide range of speeds and the valve operation is typically controlled by cain lobes. It is known that the valve operation is optimized at a certain speed. One attempt to vary the optimum speed point is by using a two step method. However, this is not the optimal method for controlling valves. All increase in the range of control of the valves would improve operation of the engine over a wider speed range.
Current engines operate with a splash type of lubrication system for piston rings. The oil is either sprayed or splashed onto the sides of the cylinder and connecting rod bearing to provide lubrication and removal of heat. Special oil rings are used to help to distribute an oil film on the cylinder walls. During start-up, damage can be done to the cylinder walls because the oil tends to drain off the walls while the engine is not in use. This can cause dry spots which can shorten the life of the engine. Also, this operation limits the effective use that can be made by the bottom movement of the piston. Thus, there is a need to develop a lubrication system that eliminates the dry spot defects inherent in present piston ring lubrication and also increase the effective use of the bottom movement of the piston.
It is known that the sliding friction of piston rings on cylinder walls of present combustion engines is one of the major friction loss generators in the engine. Also, present liquid lubricants can only operate at low to moderate temperature levels and are known to burn in extremely hot combustion cylinders which causes pollution emission problems. It is also known that if the engine parts could operate at elevated temperatures, better efficiency can be expected. Thus, there is a need to use a lubricant with a much higher temperature of degradation that also provides a minimum amount of faction.
Many other concepts have been used in the prior art to improve efficiency of combustion engines. Among these are the use of regenerative braking. Regenerative braking is the slowing of the engine by employing it as a compressor to compress air. Another method of improving efficiency is the use of the heat of exhaust to beat the cooler temperature compressed air before combustion. Add-on bottoming cycles are sometimes used to recapture some of the exhaust heat to improve efficiency; however, these devices increase the cost of the engine and are difficult to match to the primary power source. There is a need to perform this function within an combustion engine. Efficiency has also been increased by use of an external flow of air to cool the engine block. Although each of these concepts helps to increase the efficiency of an combustion engine, there is a need in the art for an engine that combines and improves these elements and other components in an effort to isolate each thermodynamic step of the combustion process to maximize efficiency.